Mixin referenced in parent class

I'm not to shure if this is really a bug, but when extending a class with defined mixins and creating an instance of the parent class after that, the mixins of the child class is also referenced in the parent class.
The problem is, that I have to create multiple classes extended from the same parent with multiple mixins with the same reference key name but different classes. When doing so, the mixins with the same key name of all children map the the same (latest) class. I use Ext JS 4.0.2.